The present invention concerns a reactive sound attenuator for air-conditioning ducts, in particular, for air ducts in paper mills. The sound attenuator comprises at least two chambers separated from one another by means of a partition wall, which partition wall is provided with an opening or with a tube placed in the direction of flow of the air flowing through the sound attenuator, the air flowing through the opening or tube out of one chamber into the other.
Ever stricter requirements are imposed on suppression of noise in the environment. One important source of noise consists of the intake and exhaust air pipes for ventilation in connection with various industrial plants and other large buildings, through which pipes especially the noise of flowers is spread into the environment. The blowers are usually chosen on the basis of the quantity of air produced by them, and attention is frequently not paid to the noise produced by them. The noise produced by the blowers has quite a wide spectrum, which also imposes particular requirements on the noise suppression.
Regarding noise suppression, paper mills are particularly demanding, because the ventilation of the paper machine hall and in particular the elimination of moisture from the drying section of the paper machine require large quantities of air.
Since the noise produced by blowers has quite a wide spectrum in the intake and exhaust air ducts connected to the blowers, it is frequently necessary to use both absorptive and reactive sound attenuators. Absorptive sound attenuators operate primarily at higher frequencies; and maximum of their attenuation is at a frequency of about 1000 Hz, whereas reactive sound attenuators operate most efficiently at low frequencies, and their maximum attenuation, is, as a rule, tuned in a range of about 100-200 Hz.
For sound attenuation at low frequencies, there are various principles, whose application have been used and are used in sound attenuators, as is well known.
As is well known, reactive attenuators are attenuators for low frequencies, whose operation is based on their geometric forms. A reactive attenuator is composed of one or several chambers or tubes, and such an attenuator causes reflection of the sound energy back towards the source of sound, or reflection of the sound energy back and forth between the chambers, whereby part of the sound energy does not pass through the attenuator.
The prior art reactive sound attenuator consisting of one or several chambers is called chamber resonator. The extent of attenuation in a chamber resonator is determined by the ratio of the cross sectional area of the chamber to the cross sectional area of the related duct, and the frequencies that are attenuated are determined by the length of the chamber. The attenuation of transmission give by Equation I (below) is true when the largest transverse dimension of the chamber is smaller than 0.8.times.wavelength (L. Baranek, Noise and Vibration Control, McGraw-Hill, 1971). EQU L.sub.TL= 10 log {1+1/4(M-1/m).sup.2 sin.sup.2 k1}db (I)
wherein
L.sub.TL =attenuation of transmission (dB) PA1 m =S.sub.2 /S.sub.1 (-) PA1 k=wave number (m+1)=2.pi./.lambda. PA1 1=length of chamber (m) PA1 n=1,2,3, ... (chamber resonator) PA1 n=2,4,6, ... (tube resonator) PA1 .lambda.=wavelength (m) PA1 1.sub.chamber =chamber length (m)
S.sub.1 =cross-sectional area of duct (m.sup.2) PA2 S.sub.2 =cross-sectional area of chamber (m.sup.2) PA2 .lambda.=wavelength (m)
From the above Equation I, it is seen that the attenuation of the chamber resonator is a periodic function of k1 and receives the value 0dB when the length of the chamber is .lambda./2, .lambda., 3.lambda./2, etc. In a corresponding way, the maximum attenuation is obtained when the length 1 of the chamber is .lambda./4, 3.lambda./4, 5.lambda./4, etc.
As is well known, such a chamber resonator is called tube resonator in which a tube is installed in the partition wall that separates, for example, two chambers from one another. If the tube is installed so that its ends are placed in the middle of the chambers maximal attenuation is achieved, besides with the normal frequency of the maximum attenuation of a chamber resonator, also when the length 1 of the chamber is .lambda./2, 3.lambda./2, 5.lambda./2, etc., i.e. L.sub.TL =0 dB when 1=.lambda., 2.lambda., 3.lambda., etc.
As can be ascertained from the above, in the prior-art ordinary reactive sound attenuators, in which the partition wall between the chambers is perpendicular, i.e. at a right angle, to the flow direction, it is a problem that therein there is always a frequency of zero attenuation, i.e., a frequency at which the attenuator does not attenuate the noise at all. The frequency of zero attenuation occurs with the wavelengths as per the Equation II. EQU n.multidot..lambda./2=1.sub.chamber (II)
wherein